The present invention relates to a Group III nitride compound semiconductor device. The invention is adapted for improvement in electrodes of a Group III nitride compound semiconductor light-emitting device such as a blue light-emitting diode.
In a Group III nitride compound semiconductor light-emitting device such as a blue light-emitting diode, various proposals have been made for obtaining uniform light emission from the whole surface of the device.
For example, in Unexamined Japanese Patent Publication No. Hei. 8-340131 and Unexamined Japanese Patent Publication No. Hei. 10-117017, a p auxiliary electrode is provided radially on an upper surface of a p contact layer to attain uniformity of electric current density injected into the p contact layer. On the other hand, for example, as described in Unexamined Japanese Patent Publication No. 10-275934, a translucent electrode may be stuck on an upper surface of a p-type contact layer so that a p seat electrode is provided thereon. In this example, a p auxiliary electrode is extended from the p seat electrode along sides of the device.
Unexamined Japanese Patent Publication No. Hei. 9-97922 and Unexamined Japanese Patent Publication No. 2000-22210 have disclosed the case where an n auxiliary electrode is provided along sides of the device from an n seat electrode formed in a corner portion of the device, by way of example.
Unexamined Japanese Patent Publication No. 2000-164930 has disclosed a comb-like electrode.
According to the present inventors"" examination, it has been found that it is preferable to increase the chip size of light-emitting diodes used in a signal or the like in which high luminance is demanded and light-emitting diodes of one color are collectively used. This is because if the number of light-emitting diodes used can be reduced by increase in chip size, a circuit for evenly distributing an electric current to respective light-emitting diodes can be designed easily and simply as well as the number of steps for assembling the light-emitting diodes can be reduced to attain reduction in production cost.
Therefore, the inventors have made examination again and again to increase the chip size of light-emitting diodes. As a result, the following problems have been found.
Since the resistance of an n contact layer (a layer on which an n electrode is formed) in a light-emitting diode is relatively high, an electric current cannot sufficiently go around to a portion far from then electrode so that light emission is reduced in the portion. On the other hand, intensive light emission is obtained in a portion near the n electrode, so that light emission becomes uneven on the whole of the device. In a conventional small-size device (300 to 400 xcexcmxe2x96xa1) viewed from this point, the portion far from the n electrode was more or less dark, but was limited to a very small area so that the unevenness of light emission was substantially not a large obstacle.
If the chip size becomes large, the amount of an electric current applied to the p seat electrode must be increased when preferable current density injected per unit light-emitting area is to be kept. The current applied to the p seat electrode flows from the p seat electrode into the translucent electrode. If the amount of the current becomes large, there is a high possibility that burning (burning off the translucent electrode in a joint portion by generated Joule heat) may occur between the p seat electrode and the translucent electrode. The area of an interface between the p seat electrode and the translucent electrode is a factor for deciding the amount of the current (permissible current quantity) permitted to be injected into the p seat electrode. It is conceived that the permissible current quantity can increase as the area increases.
If one p seat electrode and one n seat electrode are used in combination when preferred current density is to be secured in an effective light-emitting surface of a large-size chip having an outermost diameter of not smaller than 700 xcexcm, there is a fear that mold resin may be burned by heat generated in a bonding wire portion or the bonding wire itself may be broken by heat unfavorably.
The invention is provided to solve at least one of the aforementioned problems. That is, in the present invention, there is provided a Group III nitride compound semiconductor device which is a device having an outermost diameter of not smaller than 700 xcexcm, wherein a distance from an n electrode to a farthest point of a p electrode is not larger than 500 xcexcm.
According to the Group III nitride compound semiconductor device configured as described above, the farthest point of the p electrode from the n electrode is with in the aforementioned distance. Hence, even in the case where the resistance of an n-type semiconductor layer is high, electrons are sufficiently injected into the farthest device portion from the n electrode (electric current is diffused). As a result, light is emitted more evenly from the whole surface of the device.
Incidentally, the current density and the luminous output of the light-emitting device have such relation that the luminous output is saturated when the current density exceeds a predetermined value. That is, even in the case where current density exceeding the predetermined value is injected, it is impossible to obtain increase in the luminous output in accordance with the current density. It is therefore preferable that current density near the predetermined value is achieved on the whole region of the device in order to achieve both high luminous output and high luminous efficiency. When the distance between the n electrode and the p electrode is defined as in the invention, the preferred current density can be obtained on the whole region of the device and, accordingly, a device excellent in luminous efficiency can be provided.
Incidentally, in this specification, the n electrode has an n seat electrode, and an n auxiliary electrode extended from then seat electrode whereas the p electrode has a p seat electrode, and a p auxiliary electrode extended from the p seat electrode. The outermost diameter of the device is the length of the longest one of lines allowed to be drawn on the device in a plan view of the device. When the device is rectangular, the outermost diameter of the device is the length of a diagonal line. When the device is rhombic, the outermost diameter of the device is likewise the length of a diagonal line. When the device is circular or elliptic, the outermost diameter of the device is the length of a line passing through the center of a circle or ellipse. As described above, the shape of the device is not particularly limited. Besides the aforementioned shapes, polygonal shapes such as a hexagonal shape, an octagonal shape, etc. may be used as the device shapes.
The upper limit of the distance between the n electrode and the p electrode located farthest from the n electrode is selected to be more preferably 400 xcexcm, further more preferably 350 xcexcm.
In the case of a rectangular chip, such configuration is preferably applied to a chip having a length of 500 xcexcm or more on one side (700 xcexcm or more in outermost diameter). In a conventional n electrode configuration, if the chip size becomes large as described above, there is fear that a portion which is darkened because it is too far from the n electrode to obtain sufficient current density may form an unacceptably large region, and that the region may appear in the central portion of the device to make the luminous form unsuitable. In the case of a rectangular chip, the length of a side is selected to be more preferably not smaller than 600 xcexcm, further more preferably not smaller than 700 xcexcm, most preferably not smaller than 800 xcexcm.
In an aspect of the invention, configuration that the n auxiliary electrode is extended from the n seat electrode to the central portion of the device is used so that the distance between any point of the p electrode and the n electrode can be selected to be in the predetermined range.
Since the n auxiliary electrode is present in the central portion of the device, the distance from the n auxiliary electrode to any corner portion of the device is kept constant. Hence, reduction in luminous output from the corner portions can be prevented.
When the n electrode has been improved in the aforementioned manner to secure uniform diffusion of current to the n-type semiconductor layer, the next problem has loomed up newly.
Also in the type in which a translucent electrode is stuck on a p-type semiconductor layer to attain diffusion of current, if the chip size is made so large that the distance from the p seat electrode or from the p auxiliary electrode becomes large, the resistance of the translucent electrode itself as a thin film cannot be ignored so that an electric current cannot be sufficiently injected into a far portion of the p-type semiconductor layer from the p seat electrode or from the p auxiliary electrode.
In an aspect of the invention, therefore, the distance from any point on the translucent electrode to the p seat electrode or the p auxiliary electrode is selected to be in a range of from 0 to 1000 xcexcm.
According to the Group III nitride compound semiconductor device configured thus, all points of the translucent electrode are within the aforementioned distance from the p seat electrode or from the p auxiliary electrode. Hence, an electric current can be sufficiently diffused to the farthest portion of the translucent electrode from the p seat electrode or from the p auxiliary electrode so as to be injected into the p-type semiconductor layer just under the translucent electrode. As a result, light can be emitted substantially evenly from the whole surface of the device. The upper limit of the distance between any point on the translucent electrode and either of the p seat electrode and the p auxiliary electrode is selected to be more preferably 500 xcexcm, further more preferably 450 xcexcm, further further more preferably 400 xcexcm, most preferably 350 xcexcm.
In the case of a rectangular chip, such configuration is preferably applied to a chip having a length of 500 xcexcm or more on one side (700 xcexcm or more in outermost diameter). In a conventional p electrode configuration, if the chip size becomes large as described above, there is fear that a portion which is darkened because it is too far from the p electrode to obtain sufficient current density may form an unacceptably large region, and that the portion may appear in the center of the device to make the luminous form unsuitable. In the case of a rectangular chip, the length of a side is selected to be more preferably not smaller than 600 xcexcm, further more preferably not smaller than 700 xcexcm, most preferably not smaller than 800 xcexcm.
In this manner, in an aspect of the invention, configuration in which the p auxiliary electrode is extended from the p seat electrode to the central portion of the translucent electrode is used so that the distance from any point on the translucent electrode to the p seat electrode or the p auxiliary electrode can be selected to be in the predetermined range.
Since the p auxiliary electrode is present in the central portion of the translucent electrode, the distance from the p auxiliary electrode to any corner portion of the translucent electrode is kept constant. Hence, reduction in luminous output from the corner portions can be prevented.
In the Group III nitride compound semiconductor device having both the n electrode and the p electrode configured as described above, it is preferable that then auxiliary electrode and the p auxiliary electrode are arranged like a comb in a plan view of the device. The device does not operate (the device does not emit light when the device is a light-emitting device) in certain portions of the n auxiliary electrode and the p auxiliary electrode. Hence, when the n auxiliary electrode and the p auxiliary electrode are arranged like a comb, the inoperative portions can be disposed as symmetrical or regular patterns in the device, so that the device can be used easily. In the case of a light-emitting device, light can be taken out evenly.
In the Group III nitride compound semiconductor device having both the n electrode and the p electrode configured as described above, it is preferable that then auxiliary electrode and the p auxiliary electrode include portions arranged in parallel with each other in a plan view of the device. The device does not operate (the device does not emit light when the device is a light-emitting device) in certain portions of the n auxiliary electrode and the p auxiliary electrode. Hence, when the parallel portions are disposed, the inoperative portions can be disposed as symmetrical or regular patterns in the device, so that the device can be used easily. In the case of a light-emitting device, light can be taken out evenly.
As the chip size increases, electric power consumed by the device increases, and the current applied between the seat electrodes accordingly increases. If one seat electrode is provided on each of p and n sides as in the conventional case, there may occur a problem that the mold resin is burned off by heat generated in the bonding wire portion or that the bonding wire itself is broken by the heat. Therefore, in another aspect of the invention, a plurality of p seat electrodes and a plurality of n seat electrodes are provided. As a result, the aforementioned problem is solved.
In the case of a rectangular chip, the preferred chip size for the provision of the plurality of p seat electrodes and the plurality of n seat electrodes is such that the length of a side is not smaller than 500 xcexcm (the outermost diameter is not smaller than 700 xcexcm). The length of a side is selected to be more preferably not smaller than 600 xcexcm, further more preferably not smaller than 700 xcexcm, most preferably not smaller than 800 xcexcm.
If the electric power consumed by the light-emitting device increases because of increase in the chip size of the light-emitting device, there arises a problem of burning between the p seat electrode and the translucent electrode in addition to the aforementioned problem. It is therefore preferable that a p auxiliary electrode is provided to extend from the p seat electrode. When the p auxiliary electrode is provided, a sufficient area can be obtained between the p seat electrode and the translucent electrode and between the p auxiliary electrode and the translucent electrode to thereby prevent occurrence of the burning. Hence, the amount of current (permissible current quantity) allowed to be applied to the p seat electrode increases, so that the amount of current required for emitting light from the whole surface of the device can be kept sufficiently.